The present invention relates to a liquid crystal display device which is used, for example, in timepieces, cellular phones, audio systems, and other electronic equipment and which allows a viewer to view a display from both the front and the back.
In recent years, liquid crystal display devices that are thin and lightweight have been widely used in portable equipment and the like. In particular, cellular phones which are required to be small in size and weight almost always employ such liquid crystal display devices. However, liquid crystal display devices are of the light-receiving type and cannot provide enough visibility in dark places, which is one of the requirements in cellular phones. This problem is often solved by installing an illumination device on the front or back of the liquid crystal display device. In general, the former illumination device is called a frontlight and the latter is called a backlight. A schematic sectional view of a front light type display device is shown in FIG. 5. A front light shown in the drawing has a light source 14 and an optical waveguide 15. Light from the light source 14 is guided downward (toward a liquid crystal display panel 1) by the optical waveguide 15 and is reflected by a reflector 16, which is provided on the back of the liquid crystal panel 1. In this way, information displayed on the liquid crystal panel 1 is made visible to a viewer. Light from the outside (external light) also enters the liquid crystal panel through the optical waveguide 15 and makes the display on the liquid crystal panel 1 visible in a similar manner. On the other hand, a back light type display device is outlined in FIG. 6. A backlight in th drawing has a light source 14 and an optical waveguide 17, and is placed below a liquid crystal panel 1. Light from the light source 14 of the backlight is reflected upward via the optical waveguide 17 to illuminate the liquid crystal panel 1. Display is thus made visible to a viewer.
As described, the optical waveguide 15 of the front light is structured to transmit light reflected by the reflector 16, whereas the optical waveguide 17 of the back light is merely capable of diffuse reflection of light and not transmission of light.
Current cellular phones have a folding structure and this has prompted employment of a sub display device in addition to a main display device in order to enable a cellular phone user to see his/her cellular phone display information such as time and reception of a call even when the phone is folded. The sub display device makes it possible for a viewer to view from the back side of the main display device. To give an example, FIG. 7 shows a structure which has a combination of a frontlight and a liquid crystal panel 1 as a main display and a combination of a backlight and a liquid crystal panel 18 as a sub display. A semi-transmissive plate 19 is provided between an optical waveguide 17 and the liquid crystal panel 18 of the backlight if necessary.
Furthermore, a structure of a single liquid crystal panel, which can be viewed both from the front side and from the back side, is known in a published application “JP-A No. 2000-132189”. In this application, a first reflector is put on a part area of the front side of the liquid crystal panel. So, this area is only observed from the back side.
The conventional structure shown in FIG. 7 needs another display device for the sub display in addition to a main display device. With the structure where the main display device and the sub display device overlap each other, the total thickness of the liquid crystal display device is large and the cellular phone or other device that has this liquid crystal device that has this liquid crystal display device is accordingly thick. Another problem of this structure relates to cost because a driver circuit and an illumination device for the sub display device are necessary aside from those for the main display device.
As described, the conventional structure cannot provide a thin and inexpensive liquid crystal display device that is capable of main display and sub display.